The present invention generally relates to an image recording apparatus, such as a laser beam printer or a digital copying machine.
In such an image recording apparatus, a laser diode is turned ON/OFF in accordance with a video signal synchronized with an image clock signal (WCLK). A laser beam emitted from the laser diode scans an image forming media (which is generally formed of a drum-shaped or belt-shaped photosensitive member) in a main scanning direction. Thereby, an image is recorded on the image forming media by the laser beam.
The laser beam emitted from the laser diode is deflected by an optical scanning system and then projected onto the image forming media. An example of the optical scanning system is a polygonal mirror having a plurality of mirror surfaces. The laser beam emitted from the polygon mirror is successively deflected by the mirror surfaces in the main scanning direction and then projected onto the image forming media, which is moved in a sub scanning direction perpendicular to the main scanning direction.
In this case, the polygon mirror deflects the laser beam at an equal angular speed so that the laser beam moves on the image recording media at a constant speed. In order to realize such a scan, an f.theta. lens is used together with the polygon mirror.
However, an f.theta. lens is a special lens which is large in size and expensive. For this reason, recently, an optical scanning system which does not use an f.theta. lens has been proposed (see Japanese Laid-Open Patent Application No. 62-32768). In an optical scanning system which odes not use an f.theta. lens, the laser beam does not move on the image forming media at a constant speed. Thus, if a frequency f.sub.k of a pixel clock used for use in the image scan is constant, the image recorded on the image recording media will deteriorate.
In order to eliminate this problem, it is necessary to change the frequency f.sub.k of the pixel clock in accordance with a change of the scanning speed of the laser beam obtained on the image recording media. More specifically, the frequency f.sub.k of the pixel clock is increased when the laser beam is moving on the image recording media at a high speed. On the other hand, the frequency f.sub.k of the pixel clock is decreased when the laser beam is moving on the image recording media at a low speed.
The frequency f.sub.k of the pixel clock is an inverse number of a time T necessary to write or read one pixel. Thus, a change of the frequency f.sub.k changes the time T. Thus, assuming that the amount of the laser beam projected on the image recording media is constant, the amount of exposure per one pixel obtained when the laser beam is moving at a high speed is different from that obtained when it is moving at a low speed. This means that the image density changes as the scanning speed changes.
Conventionally, in order to prevent the occurrence of a change of the image density, a drive current which is applied to the laser diode is changed in accordance with a change of the frequency f.sub.k of the pixel clock. With this arrangement, the amount of light emitted from the laser diode (emission power) is changed in accordance with a change of the frequency f.sub.k of the pixel signal.
Conventionally, timing signals usually labeled PCDA, CURV, LSYNC, LGATE, SYNC1 and SYNC0 are used during one scanning period. The signal PCDA indicates the scanning period of an effective printing area on the photosensitive member. The signal CURV indicates a section in which the image clock frequency is modulated and the laser diode emission power is modulated. The signal LSYNC indicates a synchronization timing at which a video signal is output to a print data processor. The signal LGATE is a scanning period during which period the print area is scanned in the main scanning direction. The signal SYNC1 indicates an edge scanning timing of the polygon mirror. The signal SYNC0 indicates a laser diode emission timing for a synchronizing signal.
FIG. 1 is a block diagram of a conventional circuit which generates the above-mentioned timing signals. A reference clock generator 70 generates a reference clock SCLK. A counter 71 counts the reference clock SCLK, and is reset by a beam detection signal DETP used for establishing the synchronization of the scanning operation. The content of the counter 71 is decoded respectively by a group of six decoders 72. The decoders decode respective count values of the counter 71 and respectively generate the timing signals PCDA, CURV, LSYNC, LGATE, SYNC1 and SYNC0.
FIG. 2 illustrates the relationship among the timing signals. As shown in FIG. 2, periods T1-T9 are fixed. If a single image recording apparatus is designed to realize different resolution levels and/or different image recording speeds and/or to change the printing area in the main scanning direction for every page, it is necessary to provide for a large number of decoders. This needs a large-size circuit and increases the production cost.
Further, light amount modulation data used for changing the amount of light emitted from the laser diode and frequency modulation data used for changing the frequency f.sub.k of the pixel clock are fixed and generated by a pull-up or pull-down operation of input terminals of an integrated circuit (IC) device or stored in a ROM. Thus, it is necessary to change the status of the input terminals or replace the ROM in order to change the amount of light emitted from the laser diode or the frequency f.sub.k of the pixel clock.